In the furniture industry and in the panel-making industry, surface finishing by means of impregnated papers have long been known. These are pressed onto particle boards or wood fiber hardboard and represent a filler or primer (priming film) or frequently also have a decorative effect (finish or also decorative film). The inadequate supply of wood tone veneers and their high price level, as well as the attitude of customers towards replacing more frequently moderately priced furniture and the demand for special optical surface effects have all contributed to a heavy use of these types of films. In order to meet the standardized requirements, of e.g. German Standard DIN 68861, Group A, with regard to surface optical properties and the resistances, these films are usually given a subsequent coating.
To simplify the overall course of the production, the impregnated and, if necessary printed papers are provided directly with a coating by the film manufacturer and supplied as so-called finish films to the furniture and panel industries.
The coating of finish films as well as of endless edges is carried out in special machines. Either raster rollers or wire doctor blades are used as lacquer applicator and metering equipment. The amount of lacquer applied usually is between 5 and 60 g/m.sup.2 and the thickness of the wet film is between 10 and 80 microns. Drying tunnels, in which the air is heated (convectors), as well as IR radiators are used to dry the lacquer. After the coating is cured the finished film is fabricated as a roll and supplied in this form to the customers in the furniture and panel industries.
This finish film, supplied as roll goods, is glued together under pressure and heat with the substrate (particle board or hardboard). The surfaces, so obtained, require no further final treatment.
It is important that the films, which have been lacquered but not yet pressed, are largely dimensionally stable, that is, they must not curl or bend which are referred to among experts as "dishing".
Temperatures of the order of 150.degree. C. to 180.degree. C. and an elevated pressure are usually employed for 5 to 30 seconds for pressing on finish films and endless edges to manufacture panels or furniture. The completed finish films and endless edges should not tend to block or to discolor. During the pressing process, as well as already while preparing the finish films and endless edges, the formation of formaldehyde and its emission should be avoided as far as possible or at least greatly reduced.
As a result of this procedure, the finish films as well as the endless edges should comply with special requirements, since not only the properties of the final coating, but also resistance to the mechanical stresses and temperature effects, which occur during the pressing process, must be ensured.
Also, after the film is pressed onto the panels and furniture parts, the surfaces obtained must have a high scratch resistance and be resistant to the conditions encountered in the household, as provided for e.g. by German Standard DIN 68861, Group A. According to this Standard, the tests are carried out with materials encountered in the home, such as acids, namely acetic and citric acids, alkalies, such as sodium carbonate and a solution of ammonia in water, alcohols, such as ethanol, various beverages, black tea, solvents, such as gasoline and acetone, fats, such as butter and olive oil, salts, mustard, disinfectants, ball point pastes, stamp ink and scouring and cleaning materials. According to the Standard DIN 68861, Part A, the surfaces, have to withstand the action of such materials for 16 hours without visible changes.
For surfaces that are to have good resistance to conditions encountered in the home and to being scratched, attempts were made in the prior art to use lacquers, which contain alkyd resins in combination either with melamine and/or urea resin as the principal vehicle.
However, the these lacquers, formulated with significant portions of urea and/or melamine resins, exhibit appreciable disadvantages when acted upon by chemicals and particularly by water and aqueous solutions, because the surface is attacked and thus no longer satisfies the requirements with respect to surface optics and resistance of the cited Standard. Moreover, due to the reaction of the vehicle components during the curing of the lacquer film, formaldehyde is emitted already during the processing of the materials. However, this emission is not finished after the processing and can continue for a long time. Appreciable amounts of formaldehyde can emerge from the lacquer, in spite of the relatively small amount of lacquer, compared to the finished, pressed particle board.
After thermal curing of the lacquer, the films, produced with the known formulations, show a strong tendency to dish and thus have problems, on the one hand, when the film is fabricated and, on the other hand, when the film is pressed. The surfaces, so produced, cannot be painted over with the existing system, so that repairs are not possible.
Lacquers suitable for coating finish films and endless edges are disclosed, for example, in German accepted patent application No. 36 10 732. These aqueous systems are based on polyacrylate resins with self crosslinking acid amide derivatives in combination with melamine resins or melamine/urea resin mixtures and blocked acids as potential curing catalysts. Such aqueous lacquers are, however, have problems with respect to wetting the substrate and also tend to form while being processed.